Recycled Polycarbonate: Mechanical, Chemical and the BPA Question Behind Both constitutes the critical evaluation pathway for polycarbonate (PC) feedstocks recovered from post-industrial and post-consumer streams. Post-industrial PC granulate from optical disc rejections, sheet trim, and water cooler bottle preforms carries a narrow molar mass distribution and predictable rheology, whereas post-consumer baled material contains poly(methyl methacrylate), polyamide, metal springs, and pressure-sensitive adhesive residues. Melt volume-flow rate measured according to ISO 1133-1:2022 at 300 °C under 1.2 kg load shifts from 10 cm³/10 min to 18 cm³/10 min after repeated extrusion cycles, indicating progressive chain scission. Near-infrared sorting systems operating between 1,500 nm and 1,700 nm separate clear PC from PET, but carbon-black-filled PC parts require mid-infrared or electrostatic separation. Sink-float separation with brine density 1.20–1.25 g/cm³ removes polyolefins and some flame-retardant HIPS fractions; the resulting flake must be washed to remove alkalinity that otherwise accelerates surface hydrolysis during drying.

Residual bisphenol A (BPA, CAS 80-05-7) in recycled PC feedstock is not solely an additive residue. It arises from unreacted monomer, thermal degradation, and hydrolytic cleavage of carbonate linkages during prior use and reprocessing. Solvent extraction followed by liquid chromatography with tandem mass spectrometry quantifies free BPA in flake; reported concentrations in post-consumer flake vary from 5 mg/kg to 150 mg/kg depending on contact history, pH, and storage temperature. Drying before extrusion is mandatory because moisture above 0.02% by weight initiates hydrolysis at melt temperatures above 260 °C. Desiccant drying at 120 °C for 4 h to a dew point below -30 °C is the minimum practical condition; failure to maintain this condition produces splay, feed throat plugging, and loss of notched impact strength exceeding 30% in one pass.

How Does Mechanical Recycling Shift Molar Mass and Rheological Response?

Mechanical recycling of PC proceeds through granulation, hot washing, drying, and controlled reprocessing on co-rotating twin-screw extruders with L/D ratios between 40:1 and 52:1. Barrel profile segmentation starts with feed zone 25–60 °C, compaction zone 180–220 °C, melting zone 260–280 °C, and metering zone 270–300 °C. Vacuum venting at -0.08 MPa to -0.09 MPa removes residual moisture and low-molar-mass degradation products. The process conflict is narrow: high melt temperature lowers viscosity for melt filtration through 200–500 mesh screens, but each 10 °C increase above 300 °C accelerates chain scission and raises MVR by approximately 3–6 cm³/10 min. The viable processing window is therefore limited to ±5 °C around the grade-specific barrel set point; excursions beyond this range yield black specks from hot spots and unacceptable melt strength variation during strand pelletizing.

Mechanical property retention after a single closed-loop post-industrial recycle is summarised by tests on injection-moulded Type 1A specimens: tensile strength per ISO 527-2:2012 drops from 65 MPa to 58–62 MPa, while notched Izod impact per ISO 180:2019 at 23 °C drops from 70 kJ/m² to 35–55 kJ/m² depending on contamination. Flexural modulus per ISO 178:2019 remains comparatively stable at 2,300–2,450 MPa, but tensile elongation at break declines from 80% to 30–50%, which is the most sensitive indicator of hydrolytic chain scission. Data from a single-pass post-consumer source should not be extrapolated without melt filtration trials because print labels, adhesives, and polyamide fragments act as nucleating agents and uncontrolled impact modifiers with lot-to-lot loading.

Rheological control is determined by capillary rheometry per ISO 11443:2021 at 300 °C and apparent shear rates 100–5,000 s⁻¹. Shear viscosity at 1,000 s⁻¹ typically falls below 150 Pa·s for post-consumer grades with MVR above 15 cm³/10 min, which limits thin-wall filling below 0.8 mm. For such low-viscosity recyclates, tooling must be rebalanced to avoid jetting and weld-line weakness; gate lands should not exceed 0.5 mm for unreinforced PC. Published data for heavily degraded PC with MVR above 25 cm³/10 min is limited, but such material is generally unsuitable for load-bearing enclosures.

On injection moulding lines producing non-food electronic housings from mechanically recycled PC, clamp force is calculated at 4–6 kN/cm² of projected area and injection pressure is held between 800 bar and 1,400 bar. Mould temperature must be maintained at 80–95 °C; lowering mould temperature to 60 °C reduces cycle time but freezes in molecular orientation and lowers notched Izod impact by a further 10–15%. Hot-runner valve gates with 1.5–2.5 mm orifice diameters are used to manage shear heating. Typical shot sizes for thin-wall monitor bezels occupy 50–70% of barrel capacity; residence times beyond 5 min produce yellowing and BPA liberation due to thermal oxidation. Dimensional stability under heat is measured by Vicat softening temperature per ISO 306:2022 method B50; mechanically recycled PC usually retains values of 138–144 °C, but contamination from ABS lowers the Vicat temperature to 118–126 °C.

Chemical Recycling Through Alkaline Hydrolysis and Methanolysis

Chemical recycling routes for PC aim to reclaim bisphenol A and carbonate precursors for repolymerization with virgin-specification quality. Alkaline hydrolysis proceeds at 120–160 °C in aqueous sodium hydroxide, consuming two moles of NaOH per carbonate repeat unit and yielding sodium bisphenolate and carbonate salts. Acid neutralisation liberates crude BPA, which is separated by solvent extraction and recrystallised. Methanolysis operates at 180–220 °C with a base catalyst and produces BPA and dimethyl carbonate in a closed loop; dimethyl carbonate is recovered by distillation at 90 °C and can be returned to phosgene-free melt transesterification processes. The selectivity of methanolysis to BPA is above 95% when the feedstock is washed, but polycarbonate blends containing 5 wt% polyesters yield ethylene glycol and dimethyl terephthalate impurities that require additional rectification.

Purification of monomeric BPA is the critical step behind the BPA question in chemical recycling. Crude BPA from post-consumer PC contains residual colour bodies, phenol, and alkylphenol impurities. Recrystallisation from toluene at 50–70 °C followed by hot filtration reduces residual phenol to below 0.1 wt%, but polycarbonate-grade BPA requires iron content below 0.5 mg/kg and colour stability below 20 Hazen under 20% aqueous HCl test. Liquid chromatography with ultraviolet detection at 280 nm can quantify free BPA in the repolymerised pellets. Values below 1 mg/kg are typical for chemical recycling plants using triple recrystallisation, whereas single-pass distillation without crystallisation may leave 20–60 mg/kg of unreacted BPA in the pellet. The exact threshold depends on the polymerisation catalyst and end-capping efficiency.

When Residual BPA Migration Becomes a Regulatory Threshold Issue

Migration of BPA from recycled PC into food simulants is evaluated under EU 10/2011 with specific migration limit 0.05 mg/kg for BPA in food or food simulant. The test uses 3% w/v acetic acid and 10% v/v ethanol as aqueous simulants; total immersion at 40 °C for 10 days is the worst-case condition for long-term storage. Under FDA 21 CFR 177.1580, polycarbonate resins are authorised for food-contact use, but BPA-based resins are not authorised in infant feeding bottles and spill-proof cups under later amendments. In the European Union, BPA is included on the REACH Candidate List as a substance of very high concern due to endocrine-disrupting properties, which imposes communication duties under Article 33 when articles contain more than 0.1 wt%. These thresholds do not automatically prohibit recycled PC in non-food technical applications, but they constrain use in food contact, toys, and medical devices where extraction and migration testing is mandatory.

Mechanical recycling does not chemically remove BPA; it redistributes the existing monomer burden and may generate additional free BPA through hydrolysis of carbonate linkages during drying and extrusion. Therefore mechanical recyclates are generally limited to non-food, industrial, and electronic applications unless a functional barrier layer is demonstrated to reduce migration below regulatory limits. Chemical recycling, by contrast, releases BPA at the depolymerisation stage, and the BPA question becomes a purification engineering problem rather than an additive migration problem. Recycled monomer used in repolymerised PC must meet the same purity specifications as fossil-derived monomer; residual phenol below 0.1 wt%, iron below 0.5 mg/kg, and free BPA below 1 mg/kg are typical gate criteria for melt transesterification.

Claims of BPA-free status on recycled PC articles require verification against defined detection limits because no uniform international test method exists for all matrices. Manufacturers may use ISO 10993-12:2021 extraction media for medical devices, but that standard is not a food-contact migration method. The most defensible analytical approach combines high-performance liquid chromatography with fluorescence detection and mass spectrometric confirmation, achieving a reporting limit of 0.01 mg/kg in polar simulants. Without such data, enforcement authorities treat the claim as misleading under ISO 14021:2016 environmental labels and declarations.

Compounding mechanically recycled PC with phosphate flame retardants and impact modifiers is performed on twin-screw extruders with side-feeder addition at 240–260 °C, because higher melt temperatures hydrolyse resorcinol bis(diphenyl phosphate) and increase mould deposit formation. A UL 94 V-0 rating at 1.5 mm is achievable with 8–12 wt% phosphate ester, but the notched Izod impact can fall below 20 kJ/m² if the modifier concentration is below 5 wt%. Glow-wire flammability index per IEC 60695-2-12:2021 at 850 °C may pass without ignition for PC-siloxane copolymers, but post-consumer blends containing PET or ABS contamination fail to extinguish and must be segregated by flake sorting. Published data for this specific recycled PC configuration is limited, particularly for UV-stabilised formulations exposed to 1,000 h Xenon-arc weathering per ISO 4892-2:2021.

Twin-Screw Extruder Configuration Limits for Post-Consumer Feedstocks

Post-consumer PC scrap is processed on co-rotating twin-screw extruders with screw diameters between 40 mm and 75 mm and L/D ratios of 40:1 to 52:1. The screw layout includes two melting zones and three vent ports; first atmospheric vent removes trapped air, second vacuum vent removes residual moisture, and third vacuum vent strips low-molar-mass aldehydes. Carbonate degradation during reprocessing releases carbon dioxide, which generates foam if the die pressure falls below 30 bar. Die plate temperature is held at 260–280 °C; strand pelletising uses water bath temperature 35–50 °C and pellet moisture after centrifugal drying below 0.05%.

A production line bottleneck is screen changer differential pressure; with 200 mesh screens, the initial pressure drop is 40–70 bar, rising to 120 bar after 4 h on heavily contaminated flake. Screen changes are therefore scheduled every 3–5 h rather than at fixed daily intervals. Pilot-scale data for post-consumer feed with printed film contamination indicates pressure drop excursions to 160 bar within 2 h; published data for this specific configuration is limited.